Metasurfaces are the two-dimensional surface counterparts of the fully three-dimensional bulk metamaterials. Metasurfaces are currently the subject of intensive research worldwide since they can be tailored to produce a wide range of optical behaviors. However, metasurfaces generally exhibit broad spectral resonances, and it is difficult to obtain narrow (i.e. high quality-factor, Q) spectral features. Attaining such high-Q features from metasurfaces would greatly expand their application space, particularly in the areas of sensing, spectral filtering, and optical modulation. Early metasurfaces were fabricated from metals and exhibited particularly broad resonances at infrared and optical frequencies as a result of Ohmic losses. Dielectric resonator-based metasurfaces were introduced to overcome these losses and have enabled, among others, wave-front manipulation and cloaking devices, perfect reflectors, and ultrathin lenses but, although absorptive losses were reduced, the metasurface resonances remained broad due to strong coupling with the external field (i.e. large radiation losses). See J. C. Ginn et al., Phys. Rev. Lett. 108 (9), 097402 (2012); I. Staude et al., ACS Nano 7 (9), 7824 (2013); A. Arbabi et al., Nat. Nano 10 (11), 937 (2015); S. Jahani and Z. Jacob, Nat. Nano 11 (1), 23 (2016); M. I. Shalaev et al., Nano Lett. 15 (9), 6261 (2015); K. E. Chong et al., Nano Lett. 15 (8), 5369 (2015); D. Lin et al., Science 345 (6194), 298 (2014); L. Y. Hsu et al., Prog. Electromagn. Res. 152, 33 (2015); P. R. West et al., Opt. Express 22 (21), 26212 (2014); and P. Moitra et al., ACS Photonics 2 (6), 692 (2015).
Recently, new strategies based on “electromagnetically induced transparency” or “Fano resonances” have been developed that show great promise for achieving high-Q resonances. See C. Wu et al., Nat. Mater. 11 (1), 69 (2012); R. Singh et al., Appl. Phys. Lett. 105 (17), 171101 (2014); C. Wu et al., Nat. Commun. 5, (2014); Y. Yang et al., Nat. Commun. 5, (2014); and W. Zhao et al., Opt. Express 23 (5), 6858 (2015). In this approach, the resonator system is designed to support both “bright” and “dark” resonances. The incident optical field readily couples to the bright resonance, but cannot couple directly to the dark resonance. Through proper design, a weak coupling between the two resonances can be introduced, allowing energy from the incident wave to be indirectly coupled to the dark resonance. The metasurface transmission and reflection spectra resulting from such an approach feature Fano resonances that can be much narrower than the traditional metasurface resonances. This approach has been demonstrated for metal-based metasurfaces at THz frequencies where Q-factors approaching 100 have been observed. See C. Wu et al., Nat. Mater. 11 (1), 69 (2012); and R. Singh et al., Appl. Phys. Lett. 105 (17), 171101 (2014).
Even more dramatic results have been achieved by applying this strategy to dielectric resonator-based metasurfaces and Q-factors approaching 500 have been demonstrated. See Y. Yang et al., Nat. Commun. 5, (2014). A common feature of the dielectric resonator-based Fano designs demonstrated thus far is the reliance on multiple, distinct, near-field coupled dielectric structures within the unit cell. See Y. Yang et al., Nat. Commun. 5, (2014); W. Zhao et al., Opt. Express 23 (5), 6858 (2015); and F. Wang et al., Opt. Mater. Express 5 (3), 668 (2015). However, reliable and repeatable control of near-field coupling requires exacting fabrication tolerances.
Further, a need remains for a rapidly tunable, narrowband filter array that can be integrated with infrared (IR) focal plane arrays for a wide range of imaging and sensing applications. Current state-of-the-art for tunable focal plane filter arrays relies on microelectromechanical systems (MEMS)-based Fabry-Perot filters which produce spectrally broad transmission pass bands. See W. J. Gunning et al., Proc. SPIE 5783, 366 (2005). Furthermore, the tunable Fabry-Perot infrared filter array requires MEMS-based motion over large distances (of order the wavelength) to tune the spectral passband. Other approaches include liquid crystal devices, which are slow, lossy, and don't achieve narrow passbands; phase change materials such as VO2, which are not continuously tunable and do not produce desirable passband spectral profiles; and metasurface arrays fabricated on stretchable membranes, which rely on the impractical tuning mechanism of stretching. See H. Zhang et al., Appl. Opt. 53, 5632 (2014); H. Kocer et al., Appl. Phys. Lett. 106, 161104 (2015), and I. M. Pryce et al., Nano Lett.10, 4222 (2010).